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FIGURE 12.16 Trans-Splicing of Trypanosome mRNA

Primary transcript #1

Splice pattern

Primary transcript #2

Exon X Intron Exon Y Intron Exon Z

Exon X Intron Exon Y Intron Exon Z

Splice pattern

Primary transcript #2

Intron Exon Y Intron Exon Z

Trans-splicing

Intron Exon Y Intron Exon Z

Two primary transcript RNA molecules are depicted. The splicing shown combines two of the exons from the first RNA with one from the second RNA to give the final messenger RNA.

Trans-splicing mRNA Exon X Exon Y Exon Z

Exon cassette selection occurs in the gene for the skeletal muscle protein troponin T. In the rat this gene has 18 exons. Of these, 11 are always used. Five (exons 4 through 8) may be used in any combination (including none used) and the final two (exons 17 and 18) are mutually exclusive, and one or the other must be chosen. This gives a theoretical mind-boggling 64 possible final mRNAs. The result is that muscle tissue contains multiple forms of this structural protein. The details of troponin splicing vary substantially among different vertebrates. Other muscle proteins quite often show similar multiple forms.

Trans-Splicing, although rare, splices together segments from two different primary transcripts. Trypanosomes are parasitic single-celled eukaryotes that cause sleeping sickness and other tropical diseases. They evade immune surveillance by constantly changing the proteins on their cell surfaces by the genetic trick of shuffling gene parts (see Ch. 19). In addition they indulge in the trans-splicing of many genes (Fig. 12.16). On the other hand, trypanosomes do not appear to have introns and so do not have normal splicing! Although it has not (yet!) been found in vertebrates, trans-splicing of segments from one RNA molecule into another also occurs in nematodes and in the chloroplasts of plant cells.

Intervening sequences that splice themselves out are occasionally found in proteins.

Inteins and Protein Splicing

Occasional intervening sequences are found that are spliced out at the protein level. Such protein splicing is rare, which is why it was only noticed relatively recently. Inteins and exteins are the protein analogs of the introns and exons found in the DNA and RNA. In other words, inteins are intervening sequences in proteins that are present when the protein is first made, but are later spliced out. The final protein is made of the exteins that are now joined together (Fig. 12.17). Inteins have been found in yeasts, algae, bacteria and archaebacteria.

Intein splicing involves no accessory enzymes. The intein segment catalyzes its own release as a free polypeptide. Certain specific amino acids must be present at the extein/intein boundaries for the splicing reaction to work. The splicing occurs in two steps, with a branched intermediate. Serine (or cysteine) must be the first amino acid of the downstream extein, as its hydroxyl group (or sulfhydryl if cysteine is used) is needed to carry the upstream extein during the branched stage (Fig. 12.18).

Usually there is just a single intein per protein, but one example is known where two inteins are inserted into the same host protein. More bizarre is the case of the extein A segment of a protein that remains after the splicing out of any inteins intein An intervening sequence in a protein—a segment of a protein that can splice itself out trans-splicing Splicing of a segment from one RNA molecule into another distinct RNA molecule trypanosome Type of single-celled eukaryotic microorganism that lives as a parasite in higher animals and causes diseases such as sleeping sickness

Exon 1 Intron Exon 2 Extein 1 Intein Extein 2

Transcription Transcription

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